No instance has no downvotes, they just hide the button in the theme like subreddits would do too. And nothing about this system prevents people from mass downvoting or upvoting a post because of group think, ask me how I know.
If you downvote my comments they do not register on my instance. It is more than the lack of a visible button, the downvotes do not federate.
Edit: to be clear, this post is on lemmy.world, so I can be downvoted here (even though I won't see it unless I browse from lemmy.world). And from lemmy.world you can appear to downvote things on instances without downvotes, but you are basically just downvoting the local copy of the thing. The downvote never gets sent through, it just lives on your instance.
If your instance removed the button, the system will register the votes, even if you can't see it. This is just more fragmentation of the experience for users fostering more confusion and alienation of new users.
Wanna check? Log in though an app and... Ta da, downvotes!
Still no downvotes. Just because some apps can show the downvotes from the api doesn't mean they are federated.
For the record, downvote me. Please. I don't care. I like not having a downvote because it doesn't allow me to downvote other people. It helps keep me in a better headspace.
Not really in my experience. I can tell when something is disliked by how few upvotes it recieves. Instead of bad posts sitting at the bottom with -20 votes, they sit at the bottom with 1 vote. Functionally the same.
Youtube removing the dislike button was more about their attempt to obfuscate public opinion on things like movie trailers, game trailers, and the various videos related to big PR decisions by companies. The companies posting these things wanted a hosting place where only positive interactions are allowed, regardless of the topic in question, and youtube accommodated them.
It is still infinitly possible to distinguish bad videos from good ones without a dislike button on youtube. But it is much more difficult now to find out how many people agree with you that a video is bad, which used to be nice to know on things like movie trailers, game trailers, and big PR decisions.
If you remove downvotes you remove the recourse users have to push down bad content. A bad post with 100 upvotes and 500 potential downvotes should never see the light of day because the community decided it's a bad post. But because no one can push it down and there isn't enough content to push up over it, now a bad post 10 people would have seen and pushed out of the system is now bothering well over 500 more people because you couldn't downvote it.
The situation you described is asinine. First of all, a post with 100 upvotes should see the light of day, even if 500 don't like it. Something that recieves a large amount of upvotes and downvotes is controversial, not a bad post or irrelevant. It also assumes there are no moderators to correct things that are truly against what that community wants to see.
For posts that are severly off topic or is against the rules in some way, that is what moderators are for.
Because if the only way you can interact with a post is by voting up or voting down, then a few problems occur. First, any instance or app that lets you use the downvotes now has a stronger voice than anyone who isn't using the downvotes. Second, it means users can not push down bad content without a consistent supply of new content to vote over it, a massive problem for small communities. And third, it fragments the community making the service harder to use.